1. Field of the Invention
This invention is directed to coating compositions, in particular, to aqueous non-yellowing polyester primer compositions.
2. Description of the Prior Art
The finishing system of choice presently being used on the exterior of automobiles and trucks comprises a clear coating applied over a pigmented base coating that is applied over a primer coating. Primer coatings that are in current use provide excellent properties, such as, stone chip resistance and good adhesion to the base coat (situated above the primer layer) as well as the substrate below the primer layer. The substrate most typically encountered is steel that has been electrocoated with an epoxy modified resin crosslinked with a polyisocyanate. Plastic substrates are also used with the plastic surface usually modified in some manner (i.e., with an adhesion promoter or flame or plasma treated) to promote adhesion.
Typical primers in current use are polyesters and polyester urethanes that are crosslinked with a melamine resin or a polyisocyanate. While these primers provide the desired level of adhesion and chip resistance, they often yellow after baking at elevated temperatures or on aging or on exposure to UV light which may penetrate through the clear and base coats depending on the amount of pigmentation used in these coats. Obviously, yellowing of the primer layer changes the overall color of the finish, particularly, when light colors, such as, whites, light grays, tans are used. Even dark colors can be adversely affected, such as, blues and greens. Also, to achieve water dispersibility of the polyester and polyester urethane used in these primers, relatively high levels of organic solvents are used resulting in the primers having a VOC (volatile organic content) in the range of 2.0-4.0 pounds/gallon (0.24-0.48 kg/L) and higher. Current air pollution standards require coating composition to have a VOC content below 2.0 pounds/gallon (0.24 kg/L).
The novel primer composition of this invention is composed of a binder of a blend of a particular polyester, dispersed acrylic polymer, various polyols and a melamine crosslinking agent. The dispersed components are neutralized and the pH of the primer composition is adjusted with a hydroxy functional alkyl amine. The resulting primer composition has excellent adhesion to both topcoats and substrates, has good chip resistance, a VOC of less than 1.0 pounds/gallon (0.12 kg/L) and does not show significant yellowing after being baked at high temperatures or on aging.